1. Field of the Invention
The present invention relates to a fuel control system for a cylinder injection type internal combustion engine such as a gasoline engine for a motor vehicle in which fuel is directly injected into a combustion chamber defined within each cylinder of the engine. More particularly, the present invention is concerned with a fuel control system for a cylinder injection type (also referred to as the direct injection type) internal combustion engine which is capable of sustaining constantly a normal ignition or firing state while carrying out effectively a two-stroke fuel injection control in a compression stroke and an expansion stroke of the engine.
2. Description of Related Art
Heretofore, in the internal combustion engine such as a gasoline engine for a motor vehicle, the cylinder injection type fuel supply system has been widely adopted for injecting the fuel directly into the combustion chambers of the individual engine cylinders.
In general, the cylinder injection type fuel supply system for the internal combustion engine is advantageous in that effects such as mentioned below can be expected.
(1) Reduction in Fuel Cost
By injecting the fuel directly into the engine cylinder during the compression stroke of the internal combustion engine, a mass of the combustible air-fuel mixture tends to be formed around and in the vicinity of the spark plug to allow stratified combustion to take place. Thus, the fuel can undergo normal combustion for a given amount of the intake air with an air-fuel ratio smaller than the theoretical air fuel ratio.
By virtue of the stratified combustion mentioned above, the adverse influence of the exhaust gas recirculation (usually called EGR for short) to the combustion can be suppressed, enabling an increased amount of exhaust gas to be recirculated. Thus, the pumping loss is reduced, whereby the fuel cost can further be enhanced.
(2) Reduction of Harmful Gas Components Contained in Exhaust Gas
As is well known in the art, in the case of the conventional extra-cylinder injection type fuel supply system for the internal combustion engine in which the fuel is injected into an intake manifold rather than the engine cylinder, a part of the fuel injected is likely to adhere to intake valves and inner walls of the intake pipes/manifold before being charged into the engine cylinder, which is of course accompanied with some delay in the fuel supply to the engine cylinder. Consequently, in the engine starting operation at a low temperature where the fuel is difficult to gasify as well as in the transient engine operation in which high-speed response is required, harmful gas components such as CO, HC and others are likely to be discharged, giving rise to a problem.
By contrast, in the case of the cylinder injection type fuel supply system, essentially no delay can take place in the fuel supply or charge into the engine cylinder, ensuring thus high accuracy for the air-fuel ratio control, which in turn means that harmful gas components contained in the exhaust gas can be remarkably diminished owing to the ideal combustion.
Furthermore, by injecting the fuel into the engine cylinder during both the compression stroke and the expansion stroke which succeeds to the compression stroke, activation of the catalyst can be promoted even at a low temperature such as encountered in the engine starting operation, whereby the gas purification efficiency can be enhanced. By virtue of this feature, reduction of the harmful gas components contained in the exhaust gas can further be promoted.
A typical one of the cylinder injection type fuel supply systems for the internal combustion engine such as described above is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 169834/1990 (JP-A-2-169834). More specifically, disclosed in the above publication is a two-stroke fuel injection type fuel supply system in which the fuel is injected into the engine cylinder in both the suction stroke and the compression stroke.
FIG. 3 is a block diagram showing schematically and generally a functional configuration of a conventional fuel control system for the cylinder injection type internal combustion engine known heretofore.
Referring to the figure, the internal combustion engine is provided with a variety of sensors for detecting various information used for controlling operation of the engine. More specifically, there are provided a key switch, an air flow sensor, a temperature sensor, a crank angle sensor (not shown) and others, which are collectively designated by a block 1. These sensors 1 serve for generating various signals concerning the key switch on/off information K, the cooling water temperature Wt, the engine rotation number or engine speed (rpm) (also referred to as the engine speed information) Ne and the intake air flow Q.
The various signals (carrying the information K, Wt, Ne and Q) are inputted to an electronic control unit (hereinafter referred to also as ECU in short) 10 to be used for arithmetically determining control parameters used in controlling the operation of the internal combustion engine. Parenthetically, the ECU 10 may be constituted by a microcomputer or microprocessor.
A fuel injector 2 electrically driven under the control of the ECU 10 is so disposed as to inject the fuel directly into a combustion chamber defined within each of the cylinders (not shown) of the engine.
The ECU 10 is composed of an engine operation state detecting means 11, an injection mode setting means 12 and an injector driving means 13.
The engine operation state detecting means 11 serves for detecting the operation state D of the internal combustion engine on the basis of the various information such as the key switch on/off information K, the cooling water temperature Wt, the engine rotation number (rpm) Ne and the intake air flow Q.
The injection mode setting means 12 serves for changing over the fuel injection modes M in dependence on the engine operation state D detected by the engine operation state detecting means 11. More specifically, the injection mode setting means 12 determines appropriately the fuel injection timings in a one-stroke fuel injection mode, e.g. in the suction stroke fuel injection or in the compression stroke fuel injection or alternatively the fuel injection timings in a two-stroke fuel injection mode in which the fuel injection is validated in both the suction stroke and the compression stroke, respectively.
On the other hand, the injector driving means 13 is designed to drive the fuel injector 2 at the fuel injection timing conforming to the fuel injection mode M validated or set by the injection mode setting means 12.
Next, referring to a flow chart shown in FIG. 4, description will be made of a fuel injection control operation carried out by the conventional fuel control system for the cylinder injection type engine shown in FIG. 3.
In the flow chart shown in FIG. 4, a routine including the steps S102 to S104 corresponds to the operation of the engine operation state detecting means 11, a routine including the step S105 to S107 corresponds to the operation of the injection mode setting means 12, and a routine including the steps S106 and S107 corresponds to the operation of the injector driving means 13.
At first, electric power supply to the ECU 10 is started in response to an engine starting operation (step S101). Then, the engine operation state detecting means 11 incorporated in the ECU 10 fetches the key switch on/off information K in the step S102, the cooling water temperature Wt in the step S103, and the engine rotation number Ne (engine speed information) in the step S104 to detect that the internal combustion engine is operating.
Subsequently, the fuel injection mode setting means 12 makes decision as to whether or not the information indicating the engine operation state D sent from the engine operation state detecting means 11, e.g. the cooling water temperature Wt, is equal to or lower than a predetermined value Wr set in advance (step S105). Parenthetically, a value corresponding to the cold state of the engine (i.e., the state prevailing upon starting of the engine) may be selected as the predetermined value.
When it is decided in the step S105 that the cooling water temperature Wt does not exceed the predetermined value Wr (i.e., when the decision step S105 results in affirmation "YES"), this means that the catalytic medium is in the cold or inactive state. Accordingly, the two-stroke fuel injection is selected as the fuel injection mode M in the step S106.
On the other hand, when it is decided in the step S105 that the cooling water temperature Wt exceeds the predetermined value Wr (i.e., when the decision in the step S105 results in negation "NO"), this means that the catalytic medium is activated. Thus, the one-stroke fuel injection mode (i.e. compression stroke injection mode) is selected as the fuel injection mode M in the step S107.
In the fuel injection mode M selectively validated in the step S106 or S107, the injector driving means 13 drives correspondingly the fuel injector 2 for effectuating the fuel supply to the engine cylinder.
At this juncture, it is assumed that the two-stroke fuel injection effectuated in the step S106 represents a fuel injection control mode in which the fuel is injected in the two successive strokes of the compression stroke and the expansion stroke.
The two-stroke fuel injection mode is performed with a view to activating as early as possible the catalyst which is in the cold or inactive state when the cooling water temperature Wt of the engine is low, for thereby suppressing discharge of harmful components or substances contained in the exhaust gas.
Finally, it is decided in the step S108 whether or not the key switch on/off information K indicates that the key switch is in the off-state (opened state). When the answer of this decision step S108 is affirmative "YES", the processing returns to the start step S101. If otherwise (i.e., when the key switch is in the on or closed state), the step S102 is resumed for fetching or reading the key switch on/off information K.
As is apparent from the foregoing description, the fuel injection control operation in the steps S102 to S107 is repeatedly executed until the key switch information K indicates that the key switch is opened (until the decision step S108 results in "YES").
The conventional fuel control system for the cylinder injection type internal combustion engine described above however suffers a problem that when the engine in the low temperature state of the engine in which the cooling water temperature Wt is lower than the predetermined value Wr inclusive, the start and stop operation of the engine is repeated at a short time interval in the two-stroke fuel injection mode (step S106) in the state where the spark plug is not sufficiently heated.
In that case, the fuel injection repeated in every expansion stroke in the two-stroke fuel injection mode will result in contamination of the spark plug with deposition of soot, giving rise to unwanted possibility that the normal ignition is impaired.
As will now be appreciated from the foregoing description, the conventional fuel control system for the cylinder injection type internal combustion engine suffers a problem that when the starting operation of the engine is repeated at a short time interval in the low temperature state of the engine in which the cooling water temperature Wt is lower than the predetermined value Wr inclusive, the fuel injection during the expansion stroke in the two-stroke fuel injection mode is performed repeatedly in the state where the spark plug is not sufficiently heated, which results in that the spark plug is deposited with soot, providing difficulty in realizing ignition and combustion of the fuel-air mixture with high efficiency and reliability.